Sonar antennaes include hydrophones at the rear of which can be placed reflectors in order to improve the reception of acoustic waves.
It is recalled that when a planar beam of acoustic waves is reflected on a plane surface there is obtained, in front of the reflector, a stationary system of waves which gives rise to pressure and velocity fields whoses nodes are spaced at (.lambda./2), .lambda. being the wave length of the acoustic vibration.
It is known that the velocity is proportional to the pressure gradient and the velocity field is therefore offset by (.lambda./4) with respect to the pressure field. The stationary pressure waves have a node or minimum value at the surface of the reflector whereas the stationary velocity waves have a peak value thereat. The hydrophones currently utilized, in combination with a reflector, are hydrophones comprising stacks of piezoelectric elements sensitive to pressure. In order to obtain a maximum sensitivity, one must therefore place these hydrophones in front of the reflector, at a distance (.lambda./4) therefrom where the first peak of the pressure field is situated. As this distance varies with wave length, it follows that the sensitivity of the hydrophone has a maximum value for a determined frequency and decreases very rapidly on both sides of this frequency. This loss of sensitivity is not very objectionable for hydrophones with narrow bands such as those which are used for receiver antennaes with active detection which receive the return echo of a target of a wave which they themselves emit. In contrast, it considerably reduces the effectivity of passive sonar receiver antennaes which seek to detect all signals coming from a possible target and therefore which can be situated in a very wide frequency band and notably in the range of low frequencies.
Also known are hydrophones with flexible blades, each composed of one thin flexible blade fixed at its periphery to an annular support and carrying on one or both faces a piezoelectric plate. The lobes of sensitivity of such hydrophones show that these are directive, therefore, more sensitive to variations of velocity than to variations of pressure.